Global warming legislations have been progressively phasing out the traditionally used refrigerant, R134a, a fluorinated greenhouse gas (F-gas) with a high Global Warming Potential (GWP) of 1,430. In fact, R134a has been banned in passenger and light commercial vehicles in the European Union (EU) (per Directive 2006/40/EC) since Jan. 1, 2017. New vehicles using R134a are no longer allowed to be registered, sold, or entered into service. The United States (US) Environmental Protection Agency also listed R134a as being unacceptable for newly manufactured light-duty vehicles beginning in Model Year (MY) 2021 with limited exceptions. Beginning in MY 2026. R134a will be unacceptable in all newly manufactured light-duty vehicles. Even more, the Australian government announced a legislative phase-down of hydrofluorocarbons (HFCs) imports in 2016 beginning Jan. 1, 2018, and targeted an 85% reduction of HFC emissions by 2036, joining the US and EU in early action to phase down HFCs.
The synthetic refrigerant, R1234yf, with a GWP of 4, has been the dominant replacement, due to its near drop-in feature. However, the price of R1234yf is significantly higher than R134a (even though it has been dropping year over year). Some Original Equipment Manufacturers (OEMs) have also raised concerns about the flammability of R1234yf. These concerns have prompted the automobile community to look for alternate refrigerants, for example, the natural refrigerant CO2 (R744), or alternate designs to the conventional direction expansion (DX) system. The secondary loop system is one of the alternate designs.
A secondary loop system uses refrigerant to cool or heat a fluid (coolant) which is circulated in the cabin heat exchanger to provide cooling or heating. Compared to a direct expansion system where refrigerant flows through the cabin heat exchanger, a secondary loop system offers certain advantages. First, due to isolation of the refrigerant circuit, for example in the engine bay, the secondary loop system allows safe use of low-cost, low-GWP refrigerants like R744 (CO2) and R152a. A R744 directed expansion system would require safety precautions to prevent leakage into the cabin while R152a is classified as flammable and normally considered unsafe for use in direct expansion systems. The compact design with fewer fittings and shorter hoses of a secondary loop system also reduces refrigerant charge and lifetime emission. As the coolant acts as thermal storage medium, the system is able to provide more comfort, and potentially better fuel economy, during stop/start and engine-off coasting. For dual HVAC and battery thermal management, a secondary loop design yields simplified plumbing and control.
The major challenges of a secondary loop system include reduced energy efficiency and added hardware such as a chiller, a coolant pump, and a reservoir. The former results from indirect heat transfer between refrigerant and air, and the latter introduces extra cost, mass, and packaging. In order to reduce the size and cost of such systems, the heat exchangers traditionally used for cooling and heating can be operated in parallel with each of the heat exchangers capable of heating or cooling dependent upon a mode of operation. The parallel connection of the heat exchangers allows smaller heat exchangers to be utilized as both may be working together dependent upon the mode of operation to achieve similar levels of cabin comfort. In a dehumidification and reheat mode of operation, the second heat exchanger may be utilized to reheat the passenger compartment where the thermal load is not significant. Even more, adjusting coolant flow rates through each of the heat exchangers allows temperature control without the need for blend doors.